The invention relates to a masking system for the masking of a cylinder bore of a combustion engine during a thermal coating procedure, and also to the use of such a masking system.
The thermal coating of cylinder running surfaces of combustion engines by means of different thermal spraying methods belongs nowadays to the state of the art and is used widely in engines for motor vehicles of all kinds, as well as in other applications. In this connection, the corresponding cylinder running surfaces are usually activated by different methods prior to the thermal coating, for example by other activating methods known per se. In this connection, substrates of lightweight metallic alloys based on Al or Mg are used most frequently.
A widely used type of engines are engines in a V-form, in other words engines which have two rows of cylinders extending parallel to one another, with the two longitudinal axes of two adjacent cylinder liners which each belong to one of the two cylinder rows being inclined relative to one another by a specific angle by which means the characteristic V-form of the engine block of a V-type engine results.
In such V-type engines the danger exists during thermal coating that during the coating of a cylinder wall of a cylinder bore of a first cylinder row vapors, for example metal vapors of the coating material, which are never to be avoided completely in thermal coating, are deposited on the cylinder wall of an adjacent cylinder of the second cylinder row. Through the deposition of the metal vapors on the relatively cold walls on the cylinder wall of the cylinder of the second cylinder row, this cylinder wall in the second cylinder row is contaminated by the metal vapors, which has among other things a negative effect on the adhesion of a coating which is to be likewise applied to this cylinder later. Moreover, contamination by unmelted particles and overspray is to be feared.
A further problem is the heating of the engine block by the thermal coating procedure. Since the difference in thermal expansion coefficient between the thermal spray layer and the substrate can be relatively high, a temperature of the substrate above 120° C., which basically means a kind of average temperature of the engine block, has a negative effect on the residual stress level of the layers, and above 150° there is even the danger that the component which is manufactured of a lightweight metal alloy, in other words the engine block, will suffer distortion of the material and thus will become unusable.
These problems become particularly clear if one considers the thermal expansion coefficients of typically used materials: Typical expansion coefficients of thermal spray layers made of iron-based alloys are for example approximately 11×10−6/° C., whereas typical thermal expansion coefficients of aluminum based substrates are approximately 23×10−6/° C. and in the case of magnesium based substrates can typically be approximately 27×10−6/° C. This means that typical thermal expansion coefficients of the substrates, in other words of the material from which the engine blocks are manufactured, are of a size of an order of magnitude more than twice that of the thermal expansion coefficients of the sprayed on thermal spray layers.
Various apparatuses are known in the prior art which attempt in particular to solve the problem of the contamination of cylinder running surfaces with the aforementioned metal vapors.
Thus apparatuses are known in which cylinder bores which are not coated are sealed with a kind of inflatable balloon which however can lead to heat accumulation and can further aggravate the above-mentioned problems with the thermal expansion coefficients. There are other systems in use in which covers for the protection of the not-to-be-coated cylinder bores are introduced through the crankcase. Ultimately the thermal problems are not completely solved here either and, something which is equally important, the use of all known systems can only be automated with difficulty or not at all, in particular in the case of V-type engines, so that the coating process is ultimately very expensive because a lot of manual work is necessary for the coating of a large number of engine blocks.